What is the difference between the solar and traditional greenhouse?

Traditional greenhouses are made either by plastic or glass. Such a greenhouse is warmed up with by sun shine during the day, but can get cold after sunset. Therefore it is sometimes necessary to use extra heaters to keep the temperature in the greenhouse high enough at nights depending on which crops you are cultivating. This however, will of course increase production costs and is only economically motivated when high value crops are cultivated.

I guess one could argue that any greenhouse is a solar greenhouse, since trapping the heat from solar energy is what any greenhouse does. But there are important differences. A solar greenhouse is constructed to allow accumulation of solar energy from the sun's heat. That is, while a traditional greenhouse simply accumulates heated air inside the greenhouse, the solar greenhouse is constructed in a slightly more sophisticated way.

There are 2 categories of solar greenhouse, active and passive. Though they meet same purpose but use different resources.

Solar Passive Greenhouse: A passive greenhouse is the simplest form of solar greenhouse and is the most appropriate for small-scale cultivation of crops. Basically, an ordinary greenhouse can be used, but complemented with heat-storing objects that will warm up at daytime and then release heat at night, hence keeping the temperature more stable than an ordinary greenhouse. The heat-storing objects can for example be large barrels of water or bales of hay. Passive solar greenhouses are often also well insulated to make the heating more efficient.

If you want to build your own passive solar greenhouse, there are many plans available free on Internet. Just search for it, but make sure you compare a few different, since the quality of the plans and set up can vary greatly. You can of course also buy one instead.

Solar Active Greenhouses: An active solar greenhouse is a larger investment, since a more sophisticated heat collection and storage system is used. One option is to force solar heated water or air through pipes in the floor to allow more efficient heating of the greenhouse. This requires the use of electricity, as the system needs a pump. The pump can by run either by solar energy (i.e. a photovoltaic solution) or by imply plugging the pump if you have electricity available. In any case, addition of active components to the greenhouse, will make it work better, but is more expensive, so it all depends on your ambitions.

Should I build an active or passive greenhouse?

Well, again, it is up to you. Generally speaking, passive greenhouses are best suited for mild climates with a plentiful supply of sunlight and active systems are needed in cold climates with limited sunlight. It also depends on what types of crops you want to grow. For example, many herbs and vegetables, such as basil, celery, dill, fennel, leaf lettuce, garlic and green onion do not require very warm temperatures and hence work well in colder climate and/or with a passive greenhouse. Tomatoes, cucumbers, broccoli and egg plant on the other hand are examples of vegetables that will require warmer temperatures to grow.

I want to build a solar greenhouse. Where do I start?

Before you can build such a solar greenhouse, you must evaluate your site. Two factors will influence your decision: the direction your house faces, and local obstructions to light, if any. Most of the heat energy associated with sunlight comes from the south during the cold months, so it is best if your house has a long, south facing wall, but it is not essential. A wall angled as much as 20 degrees from true south loses less than 5% of the heat gain from direct sunlight, even less if most of the light during fall, winter, and spring is diffused by cloudy weather. The cloudier your weather during the heating season, the less important a perfect southern orientation becomes.

So if you can, locate your greenhouse to have a long wall facing the true south, and get rid of any obstructions that you think are fine to remove (e.g. trees that you don't really need).

You also need to consider how much glazing you need and in what angle to put it. Ideally the majority of your glazing should face the winter midday sun. Depending on where you live, the angle will vary. The amount of glazing is often recommended to be at least 1 square foot for glazing for every square foot of floor space. Slightly less in a warmer climate and slightly more in a colder one.

Which glazing should I use?

These days, plastic glazing is the most common. What to remember when deciding on glazing is that the aim is for the material to allow the greatest amount of solar energy to enter into the greenhouse while at the same time minimizing energy loss. It is also important the material lets through a natural spectrum of photosynthetically active radiation.

While plastics are generally cheaper and safer than glass, the latter will last eternally if not broken. The less expensive plastic materials on the other hand has a lifespan of only a few years. More advanced materials, such as polycarbonate film, Impact modified acrylic, and corrugated high density polyethylene are all interesting alternatives to glass, though. Make sure to discuss the options with an expert if you have higher ambitions for your solar greenhouse than just growing some lettuce for fun.

Is Insulation important?

Short answer - yes! Insulation is an important part of making your greenhouse solar, since what we want is to reduce the temperature variations that occur in ordinary greenhouses.

If you want to minimize work, at least make sure you insulate the north wall of your greenhouse, and insulated to the amount recommended for ordinary house walls where you live. You can also insulate foundation walls and sometimes the east and west walls, depending on where you live.

You can also use curtains inside the greenhouse to cover the glazing at night and thereby reduce the heat loss quite significantly.

Learn More

If you want to learn more about using solar energy more efficiently for your greenhouse, as well as about other techniques to enable growing of various crops year-round, here are three interesting resources: